1. Field of the Invention
This invention relates to a method for compensating for fluctuations in an image signal caused by the nonuniformity in how a photoelectric read-out means detects light radiated out of a recording medium (i.e., light reflected by the recording medium, light which has passed through the recording medium, or light emitted by the recording medium), or the like, in an image read-out apparatus. In the image read-out apparatus, the recording medium, on which an image has been recorded, is exposed to a light beam, and light carrying information about the image is thereby radiated out of the recording medium. The light, which has been radiated out of the recording medium, is photoelectrically detected and converted into an image signal, which is made up of a series of image signal components representing the image recorded on the recording medium.
2. Description of the Prior Art
Image read-out apparatuses, wherein a recording medium, on which an image has been recorded, is exposed to a light beam, and light radiated out of the recording medium (i.e. light reflected by the recording medium, light which has passed through the recording medium, or light emitted by the recording medium) is detected in order to allow the image to be read out, have heretofore been used as, for example, scanners for plate making and input means for computers and facsimiles. The image read-out apparatuses are also used as radiation image read-out apparatuses in radiation image recording and reproducing systems, wherein stimulable phosphor sheets are used, as proposed by the applicant in, for example, U.S. Pat. Nos. 4,258,264 and 4,346,295 and Japanese Unexamined Patent Publication No. 56(1981)-11395.
Specifically, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor, which has been exposed to the radiation, is exposed to stimulating rays, such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. In the aforesaid radiation image read-out apparatuses, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is used. The stimulable phosphor sheet is first exposed to radiation, which has passed through an object, such as a human body, and a radiation image of the object is thereby stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal.
In the image read-out apparatuses described above, a photoelectric read-out means is used to detect light which is radiated out of the recording medium when it is exposed to a light beam and which carries information about the image recorded on the recording medium. By way of example, the photoelectric read-out means is constituted of a comparatively small photomultiplier and a light guide member, one edge face of which is positioned along a main scanning line on a recording medium, and the other edge face of which is positioned such that it is in close contact with the light receiving face of the photomultiplier. Alternatively, the photoelectric read-out means is constituted of a long photomultiplier which is positioned along a main scanning line on a recording medium, as disclosed in, for example, U.S. Pat. No. 4,864,134, or a line sensor which is positioned along a main scanning line.
However, with the image read-out apparatuses described above, the image signal generated by the photoelectric read-out means fluctuates, due to nonuniformity in how the photoelectric read-out means detects the light radiated out of the recording medium (i.e. light reflected by the recording medium, light which has passed through the recording medium, or light emitted by the recording medium). This nonuniformity is caused by nonuniformity in how the light is guided by the light guide member along the main scanning direction, or by nonuniformity in the sensitivity of the long photomultiplier along the main scanning direction. In cases where the aforesaid nonuniformities cause the efficiency, with which the light is detected, to worsen (i.e. cause shading to occur), it becomes impossible to accurately detect the image which was recorded on the recording medium.
In order to eliminate the aforesaid problems, as disclosed in, for example, U.S. Pat. No. 4,734,783, the applicant has proposed apparatuses for eliminating shading, wherein the characteristics of the shading are detected in advance, and the image signal, the sensitivity of the photomultiplier, or the like, is corrected in accordance with the position at which the light beam scans in the main scanning direction. Thereby, adverse effects from shading are eliminated.
In the image read-out apparatuses, rotating polygon mirrors which have a plurality of reflecting surfaces may be used as the light deflectors. A rotating polygon mirror is advantageous over a galvanometer mirror in that the speed at which the light beam is deflected can be increased, which allows an image read-out operation to be carried out quickly.
However, the rotating polygon mirror has problems in that the reflectivity fluctuates among the reflecting surfaces and the inclination with respect to the rotation axis of the rotating polygon mirror and the distance therefrom differ among the reflecting surfaces. Also, the speed at which the rotating polygon mirror is rotated fluctuates periodically, so that the speed at which the light beam is deflected differs among the reflecting surfaces of the rotating polygon mirror. As a result, the intensity of light radiated out of a surface, which is being scanned with the light beam, differs from what it should be. These problems cause the characteristics of the shading to fluctuate, and adverse effects from the shading cannot be eliminated accurately with the shading elimination methods disclosed in, for example, U.S. Pat. No. 4,734,783.
Accordingly, the applicant proposed in, for example, U.S. Pat. Nos. 4,985,629 and 5,028,783, methods for eliminating adverse effects from shading due to fluctuations among reflecting surfaces of a rotating polygon mirror, or the like, in an image read-out apparatus utilizing the rotating polygon mirror.
However, with the proposed shading elimination methods for an image read-out apparatus utilizing a rotating polygon mirror, the operation for eliminating the adverse effects from shading is carried out for each of the reflecting surfaces of the rotating polygon mirror. Therefore, a very large number of correction values must be used during the operations for eliminating the adverse effects from shading. Also, the reflecting surface of the rotating polygon mirror, which is deflecting the light beam, must be detected such that it may be clear which correction values correspond to which reflecting surface. Accordingly, the hardware and software functions for carrying out the elimination of adverse effects from shading cannot be kept simple.